Hi Ana,
With one of my worst MS relapses I had really bad back symptoms much like what you describe. My back was very stiff with pain that would only subside by laying down to rest. I also had what I call "noodle spine" and weak legs. It was in August 2002 and the first time I suspected anything was wrong.
I hope you'll call your doctor just to let him or her know what's going on. Take care...